Control systems are used extensively in industry, building and home automation, for controlling machinery and processes. Initially, a centralised control system was used, e.g. in vehicles a control system comprising only one control module, i.e. one unit in which electrical signals from all over the vehicle were gathered, and in which control signals for the different parts of the vehicle were generated. This module initially was made from cabling with switches and relays, thereafter it was made from printplates with hardware circuits, and finally a microprocessor with peripheral devices.
Fieldbus or serial control systems were then introduced. Generally, there is one centralised or master control module and the remainder of the devices on the network are slave devices. Slave devices can be actuators, sensors or transducers for instance.
In the mean time, control systems have become even more complex, and the different functions are controlled by a plurality of control modules, each with its own microprocessor. These systems may be described as distributed fieldbus or control network systems. The modules communicate with each other, for example via a CAN (Controller Area Network) and associated protocol. The modules also have their own memory, e.g. memory chips, in which, next to the necessary programs, also data is stored which is specific for that vehicle. A review of the available types of control networks is given in “Distributed fieldbus and control network systems”, by G. Schickhuber and O. McCarthy, Computing & Control Engineering Journal, February, 1997, pages 21 to 32.
A Controller Area Network (CAN) serial bus system was developed in the 1980's which has become very popular for control purposes in vehicles and is now standardised as ISO 11898 and ISO 11519. The original concept was to link three essential control units of a vehicle control system: engine control, automatic transmission control and the anti-skid braking control systems. It has also been used in machine and automation control, agricultural machinery, medical instrumentation, elevator controls, public transport systems, and industrial automation control components. A description of the CAN system is provided in “An overview of Controller Area Network”, M. Farsi, K. Ratcliff, M Barbosa, in Computing & Control Engineering Journal, June 1999, pages 113 to 120. The original CAN concept allows peer-to-peer and broadcast or multicast communications. The physical layer is defined by ISO 11898. The data link layer is defined in CAN 2.0. Higher layers are proprietary solutions, e.g. Canopen described by M. Farsi, K. Ratcliff, M Barbosa, in “An introduction to CANopen”, Computing & Control Engineering Journal, August 1999, pages 161 to 168. Vehicle control systems have become even more complex since the first three module design—a modern luxury automobile may include 10 to 15 electronic control units.
One problem identified with the original CAN was that a message did not have a guaranteed transmission time—delays could occur and were not controlled, a result of the fact that the original CAN protocol was event-triggered. Accordingly, a time-triggered modification to CAN has been proposed—see “Time-triggered controller area network”, by G. Leen and D. Heffernan, in Computing & Control Engineering Journal, December 2001, pages 245 to 256.
However, even with these improvements, when one of the modules of a distributed fieldbus or control network system becomes defective, the whole module has to be replaced by another one. By doing this, the specific data is lost, and must be read in again by an installer with a (portable) computer or other similar field tool. There exists a non-negligible risk that data is read in wrong or incompletely.